The invention relates generally to wind turbines, and particularly to rotating blades of wind turbines. Specifically, embodiments of the present technique provide a system and method to passively attenuate wind loads on the rotating blades of wind turbines.
Wind turbines are regarded as environmentally safe and relatively inexpensive alternative sources of energy that utilize wind energy to produce electricity. A wind turbine generally includes a rotor having multiple blades, which transform wind energy into a rotational motion of a drive shaft. The drive shaft is rotationally coupled to a rotor of a generator, which transforms the rotational energy into electrical power.
Wind turbine blades often operate under atmospheric turbulence, resulting from disturbances in wind velocity. Such disturbances subject the wind turbine blades to aerodynamic bending and shear loads, which result in fatigue of the rotating blades. At times, these loads can be relatively high and damaging to the wind turbine blades. An important design consideration for wind turbine blades is, therefore, the ability to withstand maximum wind loading. Hence the blades should desirably be designed to tolerate the maximum bending and shear loads.
In certain systems, these high loads can be reduced by actively controlling the wind turbine system. This active control leads to lower weight and structural components of the blades, which in turns leads to lower cost blades. Load attenuation in turbine blades is generally achieved by actively regulating the pitch of the blades by twisting the blades, thereby influencing the angle of attack of the wind on the blades. A change in the angle of attack changes the bending load to which the blade is subjected. When pitch changes are sufficiently rapid, they can affect not only the average wind load on the blades, but also vibratory loads influencing fatigue life throughout the system. Generally, twisting toward feather (i.e. more closely aligned with the direction of the wind) in response to increasing winds reduces the aerodynamic loading on the blades, and hence, the rest of the wind turbine system. For example, the system may sense wind velocity and, in response, actively adjust the orientation of the wind turbine blades.
Alternatively, these high loads can be reduced by passively controlling the wind turbine system using a fiber material that is biased off-axis to the longitudinal axis of the blade. A wind turbine blade includes an outer skin disposed about structural members, such as crossbeams. For example, the crossbeams may be configured as shear webs, which have opposite side structures or spar caps. Under a wind load, the shear webs bear the shear loads, while the spar caps bear the bending loads. For passive attenuation of the wind loads, a substantial majority of fibers in the blade skin and/or the spar cap are inclined at an angle (generally between 15 and 30 degrees) to the axis of the blade, thereby feathering the blades to alleviate loading in extreme wind conditions. Unfortunately, this biased fiber lay-up material consumes additional material for the same load carrying capability of the blade. This increased material consumption is due to the angled orientation of the fibers, which are no longer oriented with the load. Accordingly, the blades are heavier and more costly to manufacture.
Accordingly, there is a need for an improved technique for passively attenuating wind loading on wind turbine blades that is relatively low cost and less bulky than existing techniques.